1. Field of the Invention
The present invention relates to an oscillator formed of multiple cascaded high frequency resonator stages including at least one high frequency resonator, such as a surface acoustic wave resonator (SAWR), and signal amplification, and a method of generating at least one high frequency oscillator signal having reduced vibration sensitivity and phase noise and improved oscillator loop transmission response group delay.
2. Description of Related Art
Many application exist for oscillators including, for example, radar applications wherein the oscillator generates the transmitter and receiver local oscillator (L.O.) drive signals in the radar system. Applications such as radar systems have low noise requirements since low noise transmitter and receiver L.O. signals improve detection of low-cross section and low-velocity targets.
When used as oscillator frequency control elements, acoustic resonators such as a quartz crystal bulk acoustic wave resonators (BAWR) and surface acoustic wave resonators (SAWR) impart a certain amount of self noise to the oscillator output signals. The self noise is due in large part to imperfections in the crystal and the electrode metalization used to form a resonator from the crystal. Additionally, this self noise is dependent upon the amount of external vibration imparted to the resonator. In airborne or shipborne applications, the vibrations experienced by such crystal based resonators can increase noise levels to where oscillator signal spectrum degrades by as much as 40 dB.
The self noise of an oscillator generally includes amplitude modulated noise components and frequency or phase modulated noise components from the resonator and an amplifier used in the sustaining stage portion of the oscillator. The amplitude modulated noise components are easily accounted for; consequently, most noise reduction techniques focus on reducing the frequency or phase modulated noise components.
FIG. 9 illustrates the frequency spectrum of an output signal having a frequency f.sub.0 from a conventional oscillator having a crystal resonator. The sloping and horizonal lines occurring symmetrically on either side of the line at frequency f.sub.0 (referred to as "the carrier") represent the noise due to frequency or phase modulation of the carrier signal. The sloping portion of the self noise is the near-carrier noise, while the flat portion of the noise further from the carrier is the phase noise floor level or white noise level. The self noise changes from the near-carrier noise to the phase noise floor level at an offset frequency f.sub.m .apprxeq.1/2 .pi..tau., where .tau. is the oscillator loop group delay in seconds, (i.e., the 3 dB bandwidth). The slope of the near-carrier noise measured in dBc (e.g., 30 dB per decade) is referred to as the flicker-of-frequency noise. In the typical oscillator, an amplifier amplifies the signal appearing at the output of the resonator. The input signal level to the amplifier and the noise figure of the amplifier substantially determine the oscillator output signal phase noise floor level.
An oscillator may be configured and/or analyzed as a positive feedback loop circuit in which the sustaining stage contains the feedforward amplifier, and the resonator is installed in the positive feedback path from the amplifier output to the amplifier input. In such circuits, a necessary condition for the oscillation is the maintenance, at the oscillator operating signal frequency, of 2 .pi.N radians or 360N degrees total closed loop phase shift, where N is an integer. Since the resonator is the primary frequency-determining element in the oscillator, a change in the resonator frequency (due, for example, to resonator exposure to vibration and/or resonator self-noise) is accompanied by a near-identical change in the oscillator signal frequency.
When the amplifier portion of the oscillator circuit experiences a phase change (due, for example, to amplifier flicker-of-phase and white phase noise), the oscillator must change frequency in order to maintain constant loop phase shift. The amount of oscillator signal frequency perturbation (i.e., FM noise) resulting from a given amount of amplifier phase perturbation (i.e., PM noise) is a function of the oscillator loop phase versus frequency slope or group delay. The loop group delay is almost entirely a function of, and is proportional to, the resonator loaded Q. Thus, the resonator Q can be considered a measure of the resonator's ability to minimize the conversion of amplifier PM noise to oscillator signal FM noise. This explains why lower noise signals are generated using oscillators employing high Q resonators (i.e., resonators yielding high values of loop group delay). The effect of the conversion of oscillator amplifier phase noise to oscillator signal frequency noise is most pronounced in the near-carrier portion of the oscillator signal spectrum and occurs primarily in the region either side of the carrier signal within the resonator transmission response passband.
A comparison between conventional BAWR and SAWR oscillator performance shows that, in general, SAWR oscillators operate at higher frequencies and exhibit lower phase noise floor levels. BAWR oscillators, on the other hand, generally exhibit lower near-carrier and flicker-of-frequency noise levels, and are less sensitive to vibration. These oscillator performance differences are directly related to corresponding differences in resonator characteristics. With regard to SAWR oscillators, low phase noise floor levels result from a relatively higher SAWR drive level capability. For instance, a conventional SAWR may have a maximum drive level of 20-100 mW, while a conventional BAWR may have a maximum drive level of only 1 mW.
Of further interest, the product between the frequency and the resonator Q, referred to as the frequency-Q product, remains fairly constant for quartz BAWR and SAWR. Since BAWR oscillators operate at lower frequencies (e.g., HF and VHF) than do SAWR oscillators (e.g., UHF), BAWR oscillators have proportionally higher Qs. These lower Qs and/or relatively higher resonator short-term frequency instability for SAWR contribute to the relatively higher flicker-of-frequency and near-carrier noise levels in SAWR oscillators.
U.S. Pat. No. 4,851,790 entitled "CRYSTAL CONTROLLED OSCILLATOR EXHIBITING REDUCED LEVELS OF CRYSTAL-INDUCED LOW FREQUENCY NOISE, VIBRATION SENSITIVITY AND CIRCUIT TEMPERATURE RISE", issued Jul. 25, 1989, by Driscoll (the inventor of the subject application), describes a technique for improving quartz crystal oscillator vibration sensitivity, near-carrier noise level, and phase noise floor level through the use of multiple, series connected resonators. In one embodiment of U.S. Pat. No. 4,851,790, four 80 Mhz, 3rd overtone, SC-cut resonators are connected in series, and have a net effective Q identical to that of a single resonator. This series arrangement of the BAWRs improved the near-carrier and phase noise floor levels, and the vibration sensitivity of the oscillator.
Unfortunately, applying this technique to higher frequency resonators, such as SAWR, proved difficult using current technology. To minimize the phase noise floor level, each SAWR would require a drive level on the order of 100 mW. Accordingly, combining four SAWR in series would require the use of a sustaining stage amplifier providing a drive level of more than 400 mW, and exhibiting an exceptionally low flicker-of-phase noise. Current technology does not provide amplifiers which meet these requirements. Furthermore, crystal resonators such as BAWR and SAWR have a certain amount of parasitic capacitance. As is well known, the lower the operating frequency of the resonator, the easier (i.e., the more coarsely) the parasitic capacitance can be tuned out. At the high frequency operating level of, for example SAWRs, tuning out the parasitic capacitance proves quite difficult since a small degree of mistuning has a much more detrimental effect at high frequency. As a result, cost and manufacturing factors prohibit exact tuning out of the parasitic capacitance of high frequency crystal resonators. Use of variable reactance (i.e., trimmer capacitors) to accomplish exact tuning is not practical due to their vibration sensitivity.